The pathogenesis of autoimmunity in New Zealand mice is still uncertain; however, advances are being made. Genetic, viral and immunologic factors appear to be involved. The immunologic factors are complex. A very early defect is a loss of thymic regulatory or suppressor cells. These cells may keep the immune system in check and when deficient allow autoimmune responses to occur. We are trying to separate and characterize these cells with an aim toward understanding the precise cellular defect.